Up-regulation of cholecystokinin in primary sensory neurons is associated with morphine insensitivity in experimental neuropathic pain in the rat

Oestrogen modulates cholecystokinin: opioid interactions in the nervous system

Responses of the nervous system to introceptive and extroceptive inputs depend upon the state of the brain. Oestrogen has the ability to modulate brain state and dramatically alter interactions among neural circuits to influence an organism's responses to given stimuli. Cholecystokinin (CCK) and endogenous opioid peptides (EOP) have a wide and parallel distribution in the nervous system. Their reciprocal interactions regulate a diverse physiology including reproduction, cortical function and nociception. The actions of CCK and EOP are diametrically opposed, in many regions. For example, when opioids inhibit reproductive behaviour or nociception, CCK facilitates. Because oestrogen is a powerful regulator of the expression of CCK and EOP, we examined whether oestrogen-state also modulated the interactions of these neuropeptides. In this paper we present new data and review previous work that demonstrates oestrogen modulation of functional CCK-opioid interactions that regulate reproductive behaviour, cortical function and nociception.

The role of spinal cholecystokinin in chronic pain states

It is well established that cholecystokinin (CCK) reduces the antinociceptive effect of opioids. The level of CCK and CCK receptors, as well as CKK release, exhibits considerable plasticity after nerve injury and inflammation, conditions known to be associated with chronic pain. Such altered CCK release coupled in some situation with changes in CCK receptor levels may underlie the clinical phenomenon of varying opioid sensitivity in different clinical pain conditions. In particular, neuropathic pain after injury to the peripheral and central nervous system does not respond well to opioids, which is likely to be caused by increased activity in the endogenous CCK system. CCK receptor antagonists may thus be useful as analgesics in combination with opioids to treat neuropathic pain.

A phase 1 study of the cholecystokinin (CCK) B antagonist L-365,260 in human subjects taking morphine for intractable non-cancer pain

To investigate the safety and tolerability of L-365,260 in human subjects taking morphine for intractable pain. An open label study of nine adult subjects. Two doses of L-365,260 were administered to all subjects separated by a 4 h interval (three received 10 mg, three 30 mg and three 60 mg). Haemodynamic and respiratory variables were recorded from immediately prior to first drug administration to T + 600 min. In addition, continuous electrocardiogram (ECG) monitoring and serial 12 lead ECGs were recorded along with pain and side effect measurements. No major side effects were observed. L-365,260 was well tolerated. No abnormalities in blood pressure, heart rate, respiratory rate or ECG measurements were recorded. Minor side effects were observed. L-365,260 can be safely administered at the doses investigated to human subjects receiving morphine for intractable pain.

Resolution of (RS)-proglumide using lipase from Candida cylindraceae

Proglumide is used in the treatment of neuropathic pain. It acts by inhibiting peptide cholecystokinin (CCK). Neural injury produces an elevation in plasma CCK. Proglumide has been also shown to augment the analgesic effect of sustained release morphine in neuropathic pain. Currently proglumide is administered as a racemic mixture. In the present study, an attempt is made to separate the racemic mixture of the drug using lipase obtained from Candida cylindracea by stereoselective esterification. Enzymatic stereoselective esterification was carried out in organic solvents. The resolution was studied using a chromatographic column with a chiral support and mass spectrometry. The reaction conditions for stereoselective esterification including amount of substrate, amount of enzyme, alcohol, solvent and temperature were optimised during the present investigation. Butanol and hexanol were found to be suitable for formation of S and R esters, respectively. Hexane was the best solvent for esterification and the optimum temperature was found to be 30 degreesC.

Neuropeptides in neuropathic and inflammatory pain with special emphasis on cholecystokinin and galanin

Neuropeptides present in primary afferents and the dorsal horn of the spinal cord have an important role in the mediation of nociceptive input under normal conditions. Under pathological conditions, such as chronic inflammation or following peripheral nerve injury, the production of peptides and peptide receptors is dramatically altered, leading to a number of functional consequences. In this review, the role of two neuropeptides that undergo such altered expression under pathological conditions, cholecystokinin (CKK) and galanin, is reviewed.

Increased level of cholecystokinin in cerebrospinal fluid is associated with chronic pain-like behavior in spinally injured rats

Cholecystokinin (CCK) is a physiological antagonist of opioid-mediated antinociception and may be involved in some chronic pain states where opioids have reduced effect. We have previously shown in a rat model of central neuropathic pain after spinal cord injury that blockade of CCK-B receptors lead to marked pain relief. In the present study, we showed that spinally injured rats exhibiting chronic pain-like behaviors (aversive reaction to innocuous mechanical and cold stimulation) had significantly elevated level of CCK-like immunoreactivity in cerebrospinal fluid compared to normal rats or spinally injured rats which did not exhibit pain-like behaviors. The increased level of circulating CCK in the cerebrospinal fluid may thus contribute to the maintenance of chronic pain in these rats by reducing the endogenous inhibitory tone provided by opioid peptides and may be involved in the phenomenon of opioid insensitivity.

Morphine-induced in vivo release of spinal cholecystokinin is mediated by delta-opioid receptors--effect of peripheral axotomy

Morphine and other opioid agonists induce spinal in vivo release of cholecystokinin (CCK), a neuropeptide with anti-opioid properties. However, so far the opioid receptor subtype responsible for this effect has not been determined. In the present in vivo microdialysis study, the morphine-induced release of cholecystokinin-like immunoreactivity (CCK-LI) in the dorsal horn was completely blocked by the delta-opioid antagonist naltrindole (10 microM in the perfusion fluid). Neither the mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr amide (CTOP; 10 microM in the perfusion fluid), nor the kappa-opioid receptor antagonist nor-binaltorphimine (nor-BNI); 10 microM in the perfusion fluid) had any significant effect in this respect. In addition, systemic administration of the delta-opioid receptor agonist BW373U86 (1 mg/kg, s.c.) and spinal administration of the delta(2)-opioid receptor agonist, Tyr-D-Ala-Phe-Glu-Val-Val-Gly amide ([D-Ala(2)] deltorphin II) (1 microM in the perfusion fluid) induced a significant increase of the CCK-LI level. The effect of BW373U86 on spinal CCK-LI release was completely blocked by spinal administration of naltrindole. The mu-opioid receptor agonist [D-ala(2)-N-Me-Phe(4)-Gly(5)-ol]-enkephalin (DAMGO) (1 microM in the perfusion fluid or 1 mg/kg, s.c.) failed to alter the CCK-LI level. Peripheral nerve lesions have previously been shown to down-regulate mu- and delta-opioid receptors in the dorsal horn, to increase the gene-expression of CCK and CCK-receptor mRNA in dorsal root ganglion neurons and to alter the potassium-induced spinal CCK-LI release. After complete sciatic nerve transection, administration of the two selective delta-opioid receptor agonists induced a significant release of CCK-LI, which was comparable to controls. In contrast, neither systemic nor spinal administration of morphine and DAMGO altered the spinal CCK-LI release in axotomized animals. The present data indicate that the delta-opioid receptor mediates morphine-induced CCK-LI release in the spinal cord.

Cholecystokinin-8 promotes recovery of sympathectomy induced by 6-hydroxydopamine in adult mice

We used an experimental model of sympathetic neuropathy to investigate the effects of intraperitoneal cholecystokinin-8 (CCK-8) administration on the recovery of injured peripheral neurones. After treatment of adult mice with 6-hydroxydopamine (6-OHDA), which known to induce peripheral sympathectomy, nerve growth factor (NGF) in peripheral tissue first increased and then rapidly decreased to baseline levels. Following this observation, sympathectomised mice were treated with CCK-8 starting when the NGF levels lowered toward the control value. Our results show that injections with 8 nmol/kg of CCK-8 promote not only recovery of noradrenergic innervation but also NGF and neuropeptide Y (NPY) synthesis in peripheral tissue. This latter observation suggests that the effect of CCK-8 might be mediated through the stimulation of NGF synthesis.

Effects of Leukemia Inhibitory Factor on Galanin Expression and on Axonal Growth in Adult Dorsal Root Ganglion Neurons in Vitro

Synthesis of leukemia inhibitory factor (LIF) is increased in lesioned peripheral nerves and it is thought that this may cause increased expression of galanin (GAL) in axotomized dorsal root ganglia (DRG) neurons and also to promote axonal regeneration. We therefore compared effects of LIF and nerve growth factor (NGF) on galanin expression and axonal growth using cultured intact DRGs of adult mice. In control lumbar DRGs cultured for 3 days, only 16% of neurons were immunoreactive for GAL, but this was increased to 38% in preparations cultured with LIF. NGF by itself had no effect on GAL expression, but the proportion of GAL-positive neurons in cultures incubated with LIF and NGF together (22%) was less than that observed in DRGs cultured with LIF alone. Similar results were obtained using thoracic DRGs. In collagen gels, NGF caused marked increases in the numbers and lengths of outgrowing axons as observed in previous studies. In contrast, LIF did not stimulate axonal outgrowth but increased the proportions of axons which were immunoreactive for GAL. The results indicate that expression of LIF in lesioned nerves may affect expression of neuropeptides such GAL rather than stimulating axonal regeneration.

Non-opioid actions of lamotrigine within the rat dorsal horn after inflammation and neuropathic nerve damage

Some opioid-resistant pain conditions can be alleviated by voltage-dependent Na(+) channel blockers such as lamotrigine. The mu-opioid-receptor agonist morphine can modulate cation entry into cells to affect overall cellular excitability, an effect which can in turn be endogenously antagonised by the neuropeptide cholecystokinin (CCK). However, lamotrigine may also modulate cellular excitability by non-specifically blocking voltage-dependent ion channels. We have looked for interactions of lamotrigine with the opioid/CCK pathway within the spinal dorsal horn, to rule out the possibility that lamotrigine may attenuate nociceptive responses via actions on this pathway. Both lamotrigine and the mu-opioid agonist DAMGO inhibited mustard oil-evoked cell firing by approximately 50% compared with control levels. Co-application of CCK8S reversed DAMGO-, but not lamotrigine-induced inhibition of cell firing and this reversal was prevented with the selective CCK(B) receptor antagonist PD 135158. Although lamotrigine inhibited both brush- and cold-evoked cell firing in neuropathic animals, lamotrigine inhibition of mustard oil-evoked cell firing in the same animals was not significantly greater than that observed in controls. These results suggest that the antinociceptive properties of lamotrigine within the spinal dorsal horn are unlikely to be mediated via interactions with the opioid/CCK pathway.

Opioid poorly-responsive cancer pain. Part 2: basic mechanisms that could shift dose response for analgesia

Basic research in experimental pain models may illuminate the phenomenon of cancer pain that is poorly responsive to opioid drugs. Research findings can be valuable in formulating new strategies in clinical practice. This review evaluated experimental observations in terms of the events that occur in cancer patients receiving opioid therapy for pain.

Changes in spinal cholecystokinin release after peripheral axotomy

The gene expression of cholecystokinin (CCK), a neuropeptide with anti-opioid properties, has been reported to be upregulated in some primary sensory neurons after a peripheral nerve lesion. We have recently demonstrated that the upregulation of CCK mRNA is not accompanied by an increased potassium-evoked release CCK-like immunoreactivity (CCK-LI) 2-4 weeks after a complete transection of the sciatic nerve. The potassium-evoked release of CCK-LI at earlier and later time points has, however, not been studied. The aim of the present in vivo microdialysis study was to monitor how the basal and stimulated extracellular level of CCK in the dorsal horn of the spinal cord is affected at various time points after a complete transection of the sciatic nerve (axotomy). During the first week after transection of the sciatic nerve a tendency towards an elevation of the potassium-induced (100 mM in the perfusion fluid) release of spinal CCK-LI was observed. In contrast, no potassium-induced release of CCK-LI could be detected 2-3 weeks and 2 months after axotomy. No significant effect was observed on the basal extracellular levels of CCK-LI in the dorsal horn. The present study provides further support for the notion that the adaptive changes in the dorsal horn 2 weeks and later after a deafferentiation injury do not include an increased release of CCK.

Spinal effect of the cholecystokinin-B receptor antagonist CI-988 on hyperalgesia, allodynia and morphine-induced analgesia in diabetic and mononeuropathic rats

Since evidence points to the involvement of cholecystokinin (CCK) in nociception, we examined the effect of intrathecal CI-988, an antagonist of the CCK-B receptors, on mechanical hyperalgesia and allodynia in normal, mononeuropathic and diabetic rats,. Owing to the anti-opioid activity of CCK, it has been suggested that hyperactivity in the spinal CCK system is responsible for the low sensitivity of neuropathic pain to opioids. We therefore also evaluated the effect of the combination of i.t. CI-988 + i.v. morphine on mechanical hyperalgesia in diabetic and mononeuropathic rats using isobolographic analysis. Although ineffective in normal rats, CI-988 induced antinociceptive effects in diabetic (290 +/- 20 g with a cut-off of 750 g) and mononeuropathic (117 +/- 16 g; cut-off 750 g) rats, suggesting an involvement of the CCKergic system in neurogenic pain conditions. The combination of CI-988 and morphine showed a superadditive interaction in the diabetic rats only (477 +/- 16 g; cut-off 750 g), in comparison with the antinociceptive effect of each drug. In addition, CI-988 exhibited a weak anti-allodynic effect in mononeuropathic rats, and no anti-allodynic effect in diabetic rats. These results show the CCK-B receptor blockade-mediated antinociceptive effects and reveals the antinociceptive action of morphine in diabetic rats after CCKergic system inhibition.

Supraspinal cholecystokinin may drive tonic descending facilitation mechanisms to maintain neuropathic pain in the rat

Complete or partial spinal section at T(8) has been shown to block tactile allodynia but not thermal hyperalgesia following L(5)/L(6) spinal nerve ligation (SNL), suggesting the supraspinal integration of allodynia in neuropathic pain. In the present study, the possibility of mediation of nerve injury-associated pain through tonic activity of descending nociceptive facilitation arising from the rostroventromedial medulla (RVM) was investigated. Specifically, the actions of brainstem cholecystokinin and the possible importance of sustained afferent input from injured nerve fibers were determined using pharmacological and physiological approaches in rats with SNL. Lidocaine given bilaterally into the RVM blocked tactile allodynia and thermal hyperalgesia in SNL rats and was inactive in sham-operated rats. Bilateral injection of L365,260 (CCK(B) receptor antagonist) into the RVM also reversed both tactile allodynia and thermal hyperalgesia. Microinjection of CCK-8 (s) into the RVM of naive rats produced a robust tactile allodynic effect and a more modest hyperalgesia. CCK immunoreactivity was not significantly different between SNL and sham-operated rats. The anti-nociceptive effect of morphine given into the ventrolateral periaqueductal gray region (PAG) was substantially reduced by SNL. The injection of L365,260 into the RVM or of bupivacaine at the site of nerve injury restored the potency and efficacy of PAG morphine in SNL rats. These results suggest that changes in supraspinal processing are likely to contribute to the observed poor efficacy of opioids in clinical states of neuropathic pain. These data also indicate that the activation of descending nociceptive facilitatory pathways is important in the maintenance of neuropathic pain, appears to be dependent on CCK release, and may be driven from sustained afferent input from injured nerves to brainstem sites. Collectively, these data support the hypothesis that abnormal tonic activity of descending facilitation mechanisms may underlie chronic pain from peripheral nerve injury.

Potentiation of morphine and clomipramine analgesia by cholecystokinin -B antagonist CI-988 in diabetic rats

The aim of this study was to determine the influence of an intrathecally injected cholecystokinin-B (CCK-B) receptor antagonist, CI-988, on the analgesic effect of morphine and clomipramine in diabetic rats. Administered alone, morphine (0.1 mg/kg, i.v.) and clomipramine (3 mg/kg, i.v.) have respectively no effect and only a slight effect on vocalization thresholds to paw pressure in diabetic rats, but, when coadministered with CI-988 (0.1 microg/rat, i.t.), an appreciable antinociceptive effect was observed. This suggests that a spinal blockade of cholecystokininergic system increases the analgesia induced by morphine or clomipramine. A CCK-B receptor antagonist could thus be used to lower dosages of morphine or antidepressant drugs in the management of neuropathic pain in humans, and thereby reduce their side effects.

Neurophysiology of cancer pain

BACKGROUND

Recent basic science research has greatly added to our knowledge of pain mechanisms. Application of this knowledge to cancer pain syndromes has led to new and innovative approaches to cancer pain management.

METHODS

The mechanisms involved in the three main cancer pain syndromes (somatic, visceral, and neuropathic) are reviewed, and various therapeutic options are discussed.

RESULTS

Advances in knowledge in neurophysiology, neuroanatomy, and pharmacology have allowed a greater understanding of the peripheral and central mechanisms of pain. New drugs and interventional techniques based on this knowledge have improved the control of cancer pain.

CONCLUSIONS

Understanding the neurophysiology of cancer pain promotes use of the most appropriate palliative measures for pain control.

PD-135,158, a cholecystokinin(B) antagonist, enhances latent inhibition in the rat

The antipsychotic potential of cholecystokinin (CCK)-related compounds stems from CCK's colocalization with dopamine (DA). CCK demonstrates excitatory and inhibitory effects on DA in the mesolimbic pathway. Such diverse actions might be mediated by different receptor subtypes (CCK(A) or CCK(B)). Multiple hypotheses have emerged regarding the clinical application of CCK-based drugs. Administering selective nonpeptide antagonists within animal models relevant to schizophrenia would help delineate CCK receptor involvement. One animal model simulating a cognitive dysfunction of schizophrenia is latent inhibition (LI). An animal repeatedly exposed to a stimulus that is devoid of consequence is subsequently inhibited in making new associations with that stimulus. This reflects a process of learning to ignore irrelevant stimuli. The present study examined the effects of the selective CCK(B) antagonist PD-135,158 (0.001, 0. 01, and 0.1 mg/kg) using a conditioned suppression of drinking procedure in rats. For purposes of comparison the effects of haloperidol (0.1 mg/kg) were also investigated. PD-135,158 (0.1 mg/kg), similar to haloperidol (0.1 mg/kg), elicited a clear LI effect under conditions that did not lead to LI in control rats (low number of preexposures). These findings highlight the antipsychotic potential of CCK(B) antagonists, and further illustrate the LI paradigm's capacity to detect novel, antipsychotic-like, drug activity.

An analysis of drug interaction between morphine and neostigmine in rats with nerve-ligation injury

Intrathecal neostigmine reverses mechanical allodynia in humans and animals. The efficacy of morphine in a neuropathic pain state is still controversial. This study examines the antiallodynic interaction between morphine and neostigmine in a rat model of neuropathic pain. Rats were prepared with tight ligation of left L5-6 (fifth and sixth lumbar) spinal nerves and chronic intrathecal catheter implantation. Mechanical allodynia was measured by using application of von Frey hairs to the left hindpaw. Morphine (1, 3, 10, and 30 microg) and neostigmine (0.3, 1, 3, and 10 microg) were administered intrathecally to obtain the dose-response curves and the 50% effective dose (ED(50)) for each drug. ED(50) values and fractions of the ED(50) values (1/2, 1/4, and 1/8) were administered intrathecally in an equal dose ratio to establish the ED(50). Isobolographic and fractional analyses for the drug interaction were performed. Intrathecal morphine produced a moderate antagonism of the tactile allodynia. A morphine-neostigmine combination produced a dose-dependent increase in withdrawal threshold of the lesioned hind paw with reduced side effects. Both analyses revealed a synergistic interaction after the coadministration of morphine and neostigmine. These experiments suggest that the antiallodynic action of a morphine-neostigmine combination is synergistic at the spinal level.

IMPLICATIONS

This study indicates that, by using both isobolographic and fractional analyses, the antiallodynic effect of intrathecal morphine and neostigmine is synergistic when coadministered intrathecally. In a rat model of neuropathic pain, the intrathecal morphine produced a moderate antagonism on touch-evoked allodynia at the spinal level.

Effect of morphine on cholecystokinin and mu-opioid receptor-like immunoreactivities in rat spinal dorsal horn neurons after peripheral axotomy and inflammation

In order to further investigate the interaction between the octapeptide cholecystokinin and opioid analgesia in the spinal cord we used double-colour immunofluorescence to examine the anatomical distribution of cholecystokinin and mu-opioid receptors in the dorsal horn, as well as the effect of morphine on cholecystokinin- and mu-opioid receptor-like immunoreactivities following peripheral nerve injury and inflammation. Mu-opioid receptor-like immunoreactivity was present in 65.6% of cholecystokinin-positive neurons in laminae I and II of rat spinal cord. Conversely, 40.4% of mu-opioid receptor-positive neurons contained cholecystokinin-like immunoreactivity. Systemic application of morphine (1, 3 or 10 mg/kg; i.v.) after sciatic nerve section significantly, but reversibly, decreased mu-Opioid receptor-like immunoreactivity in the medial half of lamina II in segment L5 of the ipsilateral dorsal horn, and cholecystokinin-like immunoreactivity was also markedly reduced in the same region. These effects were dose- and time-dependent and could be prevented by naloxone preadministration. In contrast, no significant change in the pattern of distribution or intensity of mu-opioid receptor- and cholecystokinin-like immunoreactivities was observed in intact rats or during peripheral inflammation. These results provide a cellular basis for the interaction of mu-opioid receptors and cholecystokinin at the spinal level by showing a high degree of co-existence of these two molecules in local interneurons, and also show that morphine can induce rapid and short lasting effects on mu-opioid receptors after peripheral nerve injury. The results contribute to our understanding of how endogenous cholecystokinin reduces the analgesic effect of morphine.

Increased spinal cholecystokinin activity after systemic resiniferatoxin: electrophysiological and in situ hybridization studies

The present study assessed the effect of a single subcutaneous injection of resiniferatoxin (RTX), an ultrapotent capsaicin analogue, on the activity of spinal cholecystokinin (CCK) systems, by using electrophysiological and in situ hybridization techniques. Subcutaneous RTX at 0.3 mg/kg, but not vehicle, produced marked thermal hypoalgesia in rats on the hot plate and tail flick tests. Partial recovery from hypoalgesia occurred in some (<50%), but not all, RTX-treated rats after 2 weeks. The flexor reflex in response to activation of high threshold afferents was recorded 15-35 days after RTX- or vehicle-treatment. There was no obvious difference between RTX- and vehicle-treated rats in the baseline flexor reflex. Intravenous morphine at 1 mg/kg caused a depression of the flexor reflex in vehicle- and in RTX-treated rats. The reflex depressive effect of morphine was significantly briefer in RTX-treated, non-recovered rats than vehicle-treated rats. Furthermore, CI-988, a high affinity antagonist of CCKB receptors, caused a minor depression of the reflex in vehicle- and RTX-treated rats that had partially recovered, whereas the reflex depressive effect of CI-988 was significantly enhanced in RTX-treated, non-recovered rats. In situ hybridization showed that RTX treatment caused a marked and significant increase in the number of dorsal root ganglion (DRG) neurone profiles expressing CCKB receptor mRNA, whereas only a small increase was observed for CCKA receptor mRNA expressing neurone profiles. Significantly more DRG neurone profiles expressed CCKB receptor mRNA in RTX-treated, non-recovered rats compared to partially recovered rats. RTX-treatment did not influence the expression of CCK mRNA in DRGs. Since CCK functions as a physiological antagonist of morphine, it is suggested that RTX treatment enhances the activity of spinal CCK systems, leading to the reduced effect of morphine and increased effect of the CCKB receptor antagonist CI-988. This may mainly be due to upregulation of CCKB receptors in DRG neurones.

Pain facilitatory circuits in the mammalian central nervous system: their behavioral significance and role in morphine analgesic tolerance

Sensitivity to noxious stimulation is not invariant; rather, it is modulated by discrete pain inhibitory and facilitatory circuits. This paper reviews the neural circuits for pain facilitation, describes the conditions governing their environmental activation, and examines their role in an animal's behavioral repertoire. Mechanisms for pain facilitation are contrasted at both the neural and behavioral level with mechanisms for pain inhibition. In addition, the involvement of mechanisms for pain facilitation in morphine analgesic tolerance is discussed, and the implications of this involvement for accounts of the role of associative processes in analgesic tolerance are highlighted.

Cholecystokinin/opioid interactions

Cholecystokinin (CCK) acts as an anti-opioid peptide. The mechanisms of CCK-opioid interaction under normal and pathological conditions were examined with various techniques. Nerve injury induces upregulation of CCK mRNA and CCK2 receptors in sensory neurons. The involvement of CCK in spinal nociception in normal and axotomized rats was examined. The CCK2 receptor antagonist CI-988 did not reduce spinal hyperexcitability following repetitive C-fiber stimulation in normal or axotomized rats, suggesting that CCK is probably not released from injured primary afferents. With in vivo microdialysis intravenous (i.v.) or intrathecal (i.t.) morphine increased the extracellular level of CCK in the dorsal horn in a naloxone reversible manner. Morphine also released CCK after axotomy, but not during carrageenan-induced inflammation. In contrast, K(+)-stimulation failed to increase extracellular levels of CCK in axotomized rats, but did so in inflamed rats. Double-coloured immunofluorescence technique revealed partial co-localization between CCK-like immunoreactivity (LI) and mu-opioid receptor (MOR)-LI in superficial dorsal horn neurons. The presence of MOR in CCK containing neurons suggests a possible direct influence of opioids on CCK release in the spinal cord. Axotomy, but not inflammation, induced a moderate decrease in CCK- and MOR-LI in the dorsal horn. I.v. morphine further temporarily reduced CCK- and MOR-LIs in axotomized, but not in normal or inflamed, rats. While the effect of morphine on CCK-LI can be interpreted as the result of increased CCK release, the effect on MOR-LI may be related to changes in the microenvironment of the dorsal horn induced by nerve injury.

Excitability of spinal cord and gracile nucleus neurons in rats with chronically injured sciatic nerve examined by c-fos expression

Low-threshold sensory pathways have been suggested to have an important role in the formation and maintenance of sensory abnormalities which are observed after peripheral nerve injury. Fos-like immunoreactive (Fos-LI) neurons are expressed in spinal cord laminae III-IV and the gracile nucleus by electrically stimulating the injured nerves at Abeta strength after sciatic nerve transection in rats. This suggests that the excitability of these neurons is increased by nerve injury. In this study, we investigated which receptors are involved in the regulation of the increased excitability in spinal and gracile nucleus neurons. The sciatic nerve of Sprague-Dawley rats (150 g) was transected 7 days before the experiment day. The rats were administered morphine, muscimol, baclofen, MK-801, CNQX, N(G)-nitro-L-arginine methyl ester hydrochloride (L-NAME) or clonidine i.p., and then electrically stimulated at 0.1 mA to the proximal region to the nerve injury site under urethane anesthesia. Two hours after the stimulation, Fos-LI expression was increased in the spinal cord dorsal horn and the gracile nucleus in control rats. Baclofen inhibited the Fos-LI expression both in the spinal cord and the gracile nucleus. Morphine inhibited only the Fos-LI expression in the posterior cutaneous (PC) nerve territory of laminae I-II, but not in the sciatic nerve (SC) territory, laminae III-IV nor the gracile nucleus. MK-801 had an inhibitory but complicated effect in laminae I-II and the gracile nucleus. The other drugs were not effective on Fos-LI expression. It is suggested that the GABA(B) receptor has a pivotal role in the regulation of Fos-LI expression after electrical stimulation to the injured low-threshold sensory fibers, and other receptors have little effect on the Fos-LI expression.

Cholecystokinin-8-like-immunoreactive fibers in rat lumbosacral autonomic regions are sexually dimorphic and altered by a reduction of androgen receptors

Cholecystokinin-8-like-immunoreactive (CCK-8-LI) fibers in laminae VII and X of the rat lumbosacral spinal cord demarcate the position of preganglionic autonomic neurons. This investigation reveals that adult male Sprague-Dawley, or King-Holtzman/Sprague-Dawley rats contain more CCK-8-LI fibers in lumbosacral laminae VII and X than adult females. Furthermore, testicular feminization mutation male rats (which lack 85-90% of their functional androgen receptors) contain fewer CCK-8-LI fibers than normal male or female rats, with the amount of CCK-8-LI being reduced to a greater extent in the sympathetic vs. the parasympathetic regions of the lumbosacral spinal cord. Thus, CCK-8-LI in testicular feminization mutation male rats has a distinctly female-like pattern. These results suggest that testosterone is a regulatory factor for CCK-8-LI fibers found in laminae VII and X of the lumbosacral spinal cord. Sexual dimorphism in lumbosacral CCK-8-LI fibers may contribute to modulating the final common pathway which differentially regulates the reproductive organs and stereotypic reproductive behavior, and may be involved with the sex differences described for pain.

Spinal effect of a neuropeptide FF analogue on hyperalgesia and morphine-induced analgesia in mononeuropathic and diabetic rats

1DMe, a neuropeptide FF (NPFF) analogue, has been shown to produce antinociception and to enhance morphine analgesia in rats after intrathecal administration. To determine whether 1DMe could correct hyperalgesia and restore morphine efficacy in mononeuropathic (MN) and diabetic (D) rats we examined the spinal effect of 1DMe in MN and D rats without and after spinal blockade of mu- and delta-opioid receptors with CTOP and naltrindole, respectively. The influence of 1DMe on morphine-induced antinociception was assessed in the two models using isobolographic analysis. Whereas 1DMe intrathecally injected (0.1, 1, 7.5 microg rat(-1)) was ineffective in normal (N) rats, it suppressed mechanical hyperalgesia (decrease in paw pressure-induced vocalisation thresholds) in both MN and D rats. This effect was completely cancelled by CTOP (10 microg rat(-1)) and naltrindole (1 microg rat(-1)) suggesting that it requires the simultaneous availability of mu- and delta-opioid receptors. The combinations of morphine: 1DMe (80.6:19.4% and 99.8:0.2%, in MN and D rats, respectively) followed by isobolographic analysis, showed a superadditive interaction, relative to the antinociceptive effect of single doses, in D rats only. In N rats, the combination of morphine: 1DMe (0.5 mg kg(-1), i.v.: 1 microg rat(-1), i.t., ineffective doses) resulted in a weak short-lasting antinociceptive effect. These results show a different efficacy of 1DMe according to the pain model used, suggesting that the pro-opioid effects of the NPFF in neuropathic pain are only weak, which should contribute to hyperalgesia and to the impaired efficacy of morphine.

CCK-B receptor: chemistry, molecular biology, biochemistry and pharmacology

Cholecystokinin (CCK) is a peptide originally discovered in the gastrointestinal tract but also found in high density in the mammalian brain. The C-terminal sulphated octapeptide fragment of cholecystokinin (CCK8) constitutes one of the major neuropeptides in the brain; CCK8 has been shown to be involved in numerous physiological functions such as feeding behavior, central respiratory control and cardiovascular tonus, vigilance states, memory processes, nociception, emotional and motivational responses. CCK8 interacts with nanomolar affinities with two different receptors designated CCK-A and CCK-B. The functional role of CCK and its binding sites in the brain and periphery has been investigated thanks to the development of potent and selective CCK receptor antagonists and agonists. In this review, the strategies followed to design these probes, and their use to study the anatomy of CCK pathways, the neurochemical and pharmacological properties of this peptide and the clinical perspectives offered by manipulation of the CCK system will be reported. The physiological and pathological implication of CCK-B receptor will be confirmed in CCK-B receptor deficient mice obtained by gene targeting (Nagata el al., 1996. Proc. Natl. Acad. Sci. USA 93, 11825-11830). Moreover, CCK receptor gene structure, deletion and mutagenesis experiments, and signal transduction mechanisms will be discussed.

Effects of peripheral axotomy on cholecystokinin neurotransmission in the rat spinal cord

Because cholecystokinin (CCK) acts as a "functional" endogenous opioid antagonist, it has been proposed that changes in central CCKergic neurotransmission might account for the relative resistance of neuropathic pain to the analgesic action of morphine. This hypothesis was addressed by measuring CCK-related parameters 2 weeks after unilateral sciatic nerve section in rats. As expected, significant decreases (-25-38%) in the tissue concentrations and in vitro release of both substance P and calcitonin gene-related peptide were noted in the dorsal quadrant of the lumbar spinal cord on the lesioned side. In contrast, the tissue levels and in vitro release of CCK were unchanged in the same area in lesioned rats. Measurements in dorsal root ganglia at L4-L6 levels revealed no significant changes in proCCK mRNA after the lesion. However, sciatic nerve section was associated with a marked ipsilateral increase in both CCK-B receptor mRNA levels in these ganglia (+70%) and the autoradiographic labeling of CCK-B receptors by [3H]pBC 264 (+160%) in the superficial layers of the lumbar dorsal horn. Up-regulation of CCK-B receptors rather than CCK synthesis and release probably contributes to increased spinal CCKergic neurotransmission in neuropathic pain.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Correlation between autotomy-behavior and current theories of neuropathic pain

The past 10 years have brought several new experimental models with which to study chronic neuropathic pain in animals. Consequently, our knowledge about the mechanisms subserving neuropathic pain in humans has improved. However, the first animal model that was used for studying this type of chronic pain was the autotomy-model which can still be considered as a useful tool for pain studies. The present review assesses some of the similarities and differences between autotomy-model and more recent models of experimental traumatic mononeuropathy. In addition, it considers some of the similarities between the results obtained in clinical studies and in autotomy studies.

Neuropathic pain in the cancer patient

Cancer presents itself in numerous ways, adding to the complexity of any pain syndrome with which it is associated. Neuropathic pain, unlike many other pain syndromes, is difficult to treat even in the absence of cancer. The combination results in a heterogeneous group of patients with a complex set of symptoms. This makes the assessment of pain, classification of syndromes, and clinical study a challenge. If the disease is nonprogressive, general principles of care are essentially the same as in those without cancer. In patients with progressive disease and more refractory painful conditions, spinal anesthetic and neurosurgical therapies must often be considered. Under such circumstances, caregivers are forced to carefully balance uncertain benefits and risks, often without the luxury of time. More careful observation and controlled trials in these patients help facilitate this challenging process.

Anti-allodynic actions of intravenous opioids in the nerve injured rat: potential utility of heroin and dihydroetorphine against neuropathic pain

Neuropathic pain has been suggested to be resistant to treatment with opiates. Such perceived lack of opioid responsiveness may be due to the dose-range over which specific opioid compounds have been studied as well as the efficacy of these compounds. Dihydroetorphine is a novel opiate that demonstrates significantly greater analgesic potency compared to morphine, and which also demonstrates diminished capacity for producing physical dependence in laboratory animals. The present study compared the intravenous (i.v.) efficacy, potency and duration of action of dihydroetorphine, fentanyl, heroin and morphine in producing anti-allodynic actions in a rat model of neuropathic pain (ligation of the L5/L6 nerve roots). All compounds produced significant anti-allodynic activity with dihydroetorphine being the most potent (A50 of 0.2 microg kg(-1), i.v.). Morphine was approximately 7440 times less potent than dihydroetorphine while heroin and fentanyl were approximately 163.5 and 6.9 times less potent in producing anti-allodynic actions. Dihydroetorphine also showed a maximal effect at 0.6 microg kg(-1) in all animals tested, while 100 microg kg(-1) was required for heroin to produce a maximal effect. Fentanyl and morphine did not elicit a maximum anti-allodynic response (74 and 76% maximum possible effect (%MPE), respectively). As expected, fentanyl showed a relatively brief duration of action (approximately 20 min at the highest tested dose), while dihydroetorphine and morphine demonstrated anti-allodynic actions for up to 45 min. Heroin had the longest duration of action, producing significant anti-allodynic effects for up to 90 min. These data show that dihydroetorphine and heroin produce potent and long-lasting anti-allodynic actions in this model. Additionally, in contrast to morphine and fentanyl, both dihydroetorphine and heroin were able to achieve a maximal response. The remarkable potency, maximal efficacy and duration of action of these compounds, particularly dihydroetorphine, suggests that these compounds may warrant further examination as potential therapeutic treatments for neuropathic pain states.

Peripheral axotomy influences the in vivo release of cholecystokinin in the spinal cord dorsal horn-possible involvement of cholecystokinin-B receptors

An increased expression of cholecystokinin (CCK) messenger RNA (mRNA) as well as CCK-B receptor mRNA in dorsal root ganglion (DRG) cells following peripheral axotomy has previously been demonstrated. In the present in vivo microdialysis study, the effect of unilateral sciatic nerve section on basal and potassium-induced release of CCK-like (CCK-LI) immunoreactivity in the rat dorsal horn was investigated. We also compared the effects of the CCK-B receptor antagonist CI988 on basal and potassium-stimulated CCK-LI release in intact animals and in chronically axotomized rats. Perfusion of the microdialysis probe with KCl (100 mM) induced a more than 6-fold increase of the extracellular level of CCK-LI in control animals. In contrast, following unilateral sciatic nerve section the same KCl stimulation failed to evoke a release of CCK-LI ipsilaterally. However, after systemic administration of CI988 (1 mg kg-1, i.v.), 100 mM KCl induced a significant increase of the extracellular CCK-LI level in axotomized rats, similar to that observed in control animals. In control animals no effect of CI988 on KCl-stimulated CCK-LI release could be detected. CI988 by itself had no influence on the extracellular CCK-LI level in either nerve injured or control animals. The present data suggest that axotomy reduces the release of CCK-like immunoreactivity in the spinal cord by a mechanism involving the CCK-B receptor binding site.

Differential release of cholecystokinin by morphine in rat spinal cord

The analgesic efficacy of opioids is reduced in neuropathic pain states and increased in inflammation. Since the neuropeptide cholecystokinin (CCK) plays a role in the modulation of opiate-induced analgesia, the morphine-mediated release of CCK in the spinal cord of rats was compared with in vivo microdialysis in normals and different pain models. The effect of systemic and intrathecal (i.t.) morphine on the extracellular level of CCK was analyzed in the spinal cord dorsal horn of halothane-anaesthetized normal rats as well as during peripheral neuropathy and inflammation. No difference was found in basal CCK level among groups. However, morphine significantly increased extracellular CCK concentration after both systemic and spinal application in intact as well as axotomized rats and this effect was naloxone-reversible in non-lesioned animals. Similar results were seen in axotomized rats. In contrast, morphine did not induce CCK release during carrageenan-induced inflammation. These data provide evidence that the ability of opiates to release CCK under different pain states may play a key role in their analgesic efficacy.

Localization and regulation of the delta-opioid receptor in dorsal root ganglia and spinal cord of the rat and monkey: evidence for association with the membrane of large dense-core vesicles

Using immunohistochemistry and immunoelectron microscopy, the localization and regulation of delta-opioid receptor-like immunoreactivity were studied in dorsal root ganglia and spinal cord of normal rat and monkey, and after peripheral axotomy. Delta-opioid receptor-like immunoreactivity was observed in many small dorsal root ganglion neurons, and in the rat most of them contained substance P and calcitonin gene-related peptide. At the ultrastructural level, delta-opioid receptor-like immunoreactivity was localized in the Golgi complex, on the membrane of the large dense-core vesicles and on the membrane of and/or inside a type of large vesicle with an interior of low electron density. The latter vesicles were often in contact with multivesicular bodies. In the superficial dorsal horn of the spinal cord, most delta-opioid receptor-positive nerve fibers contain substance P and/or calcitonin gene-related peptide, both in rat and monkey. Also, in these nerve endings delta-opioid receptor-like immunoreactivity was found on the membrane of large dense-core vesicles and on the membrane of, or in, the lucent vesicles. Occasionally, delta-opioid receptor-like immunoreactivity was observed on the plasmalemma of the terminals, particularly when the vesicles were in exocytotic contact with the plasmalemma. Peripheral axotomy induced a decrease in delta-opioid receptor-like immunoreactivity both in cell bodies in the dorsal root ganglia and in terminals in the dorsal horn. These data suggest that the delta-opioid receptor may be a constituent of the membrane of large dense-core vesicles storing and releasing neuropeptides. It is suggested that upon exocytotic release of substance P and calcitonin gene-related peptide from large dense-core vesicles, there is a transient modification of the surface of the primary afferent terminals which leads to exposure of the receptor protein so that enkephalin released from adjacent terminals can activate the receptor. The decrease in delta-opioid receptors after axotomy indicates that delta-opioid receptor-mediated inhibitory effects are attenuated at the spinal level both in the rat and monkey.

On the role of cholecystokinin in the mediation of spinal reflex excitability in intact and axotomized rats

The effects of intrathecal (i.t.) administration of cholecystokinin (CCK) on the hindlimb flexor withdrawal reflex were compared in decerebrate, spinalized, unanaesthetized rats with intact sciatic nerves or 14-26 days after unilateral transection of the sciatic nerve. The effect of CI-988, a CCK-B receptor antagonist, on the facilitation of the flexor reflex induced by conditioning stimulation (CS) of C-afferents was also examined. Intrathecal CCK induced facilitation of the flexor reflex over the dose range of 10 ng-1 micrograms in rats with intact and sectioned sciatic nerves. However, the facilitation induced by CCK in axotomized rats was significantly reduced compared with nerve intact animals. CI-988 did not block the facilitation of the flexor reflex induced by the CS and the increases in reflex magnitude during the CS train (wind-up) in rats with intact and sectioned sciatic nerves. The present results indicate that the excitatory effect of CCK on the spinal cord is reduced after axotomy, suggesting that nerve section may induce transsynaptic changes in the spinal cord leading to hyposensitivity to CCK. Furthermore, it is suggested that CCK may not play a significant role in the phenomenon of wind-up and central sensitization after repetitive C-fiber input under normal conditions or after axotomy, when expression of CCK and CCK-B receptors are increased in sensory neurons.

Tactile allodynia, but not thermal hyperalgesia, of the hindlimbs is blocked by spinal transection in rats with nerve injury

Spinal nerve ligation produces signs of neuropathic pain in rats. Different neuronal pathways may underlie the abnormal sensory responses to thermal and tactile stimuli. Here, the possibility that local circuitry in the spinal cord and/or spinal-supraspinal loops might be involved in tactile allodynia and thermal hyperalgesia of the hindpaws was investigated by transecting the spinal cord of sham-operated or L5/L6 nerve ligated rats. Spinal transection completely abolished tactile allodynia in ligated rats. Thermal nocifensive responses were present after transection in ligated and sham-operated rats. Thermal hyperalgesia of the hindpaws was not evident in spinal transected, ligated rats. Tail-withdrawal responses to tactile probing were very robust after spinal transection in both groups, demonstrating loss of descending inhibition. These observations suggest that thermal hyperalgesia of the paw seen after nerve injury involves both spinal and supraspinal circuits, while tactile allodynia depends on a supraspinal loop. This difference may reflect afferent inputs associated with different fiber types.

Down-regulation of mu-opioid receptors in rat and monkey dorsal root ganglion neurons and spinal cord after peripheral axotomy

To understand the role of opioids and their receptors in chronic pain following peripheral nerve injury, we have studied the mu-opioid receptor in rat and monkey lumbar 4 and 5 dorsal root ganglion neurons and the superficial dorsal horn of the spinal cord under normal circumstances and after peripheral axotomy. Our results show that many small neurons in rat and monkey dorsal root ganglia, and some medium-sized and large neurons in rat dorsal root ganglia, express mu-opioid receptor-like immunoreactivity. Most of these neurons contain calcitonin gene-related peptide. The mu-opioid receptor was closely associated with the somatic plasmalemma of the dorsal root ganglion neurons. Both mu-opioid receptor-immunoreactive nerve fibers and cell bodies were observed in lamina II of the dorsal horn. The highest intensity of mu-opioid receptor-like immunoreactivity was observed in the deep part of lamina II. Most mu-opioid receptor-like immunoreactivity in the dorsal horn originated from spinal neurons. A few mu-opioid receptor-positive peripheral afferent terminals in the rat and monkey dorsal horn were calcitonin gene-related peptide-immunoreactive. In addition to pre- and post-junctional receptors in rat and monkey dorsal horn neurons, mu-opioid receptors were localized on the presynaptic membrane of some synapses of primary afferent terminals in the monkey dorsal horn. Peripheral axotomy caused a reduction in the number and intensity of mu-opioid receptor-positive neurons in the rat and monkey dorsal root ganglia, and of mu-opioid receptor-like immunoreactivity in the dorsal horn of the spinal cord. The decrease in mu-opioid receptor-like immunoreactivity was more pronounced in the monkey than in the rat dorsal root ganglia and spinal cord. It is probable that there was a parallel trans-synaptic down-regulation of mu-opioid-like immunoreactivity in local dorsal horn neurons of the monkey. These data suggest that one factor underlying the well known insensitivity of neuropathic pain to opioid analgesics could be due to a marked reduction in the number of mu-opioid receptors in the axotomized sensory neurons and in interneurons in the dorsal horn of the spinal cord.

Postoperative pain and superficial abdominal reflexes after posterolateral thoracotomy

BACKGROUND

Posterolateral thoracotomy can produce stretching of/or damage to the intercostal nerves and their branches. To assess intercostal nerve impairment after operation, we measured the superficial abdominal reflexes, which are mediated, at least in part, by the most inferior intercostal nerves.

METHODS

Using electrophysiologic techniques, we made recordings from the left and right abdominal walls to study the responses evoked by mechanical stimulation of the skin after operation. In addition, we assessed postoperative pain intensity according to a numeric rating scale and recorded postoperative opioid dose.

RESULTS

We found that the patients with complete disappearance of the superficial abdominal reflexes experienced more severe postoperative pain than those in whom the reflexes were maintained. Moreover, opioid treatment was less effective in the patients with no reflexes postoperatively.

CONCLUSIONS

Our findings show a strict correlation between pain intensity after posterolateral thoracotomy and absence of abdominal reflexes. We suggest that the higher pain intensity together with the absence of reflexes may be due to intercostal nerve impairment, be it anatomic or functional, and thus to a larger neuropathic component of postoperative pain. This finding may be used as a predictor of patients with high analgesic requirements.

In mononeuropathic rats, the enhancement of morphine antinociception by L-365,260, a selective CCK(B) receptor antagonist, depends on the dose of systemic morphine and stimulus characteristics

The ability of the selective cholecystokinin(B) (CCK(B)) receptor antagonist L-365,260 (0.2 mg/kg s.c.) to modulate the antinociceptive action of relatively low doses of systemic morphine (0.1, 0.3 and 1.0 mg/kg i.v.) was evaluated using a well established rat model of peripheral unilateral mononeuropathy. Behavioural tests based on both mechanical (vocalization threshold to paw pressure) and thermal (struggle latency after immersion of the paw into a cold (10 degrees C), warm (44 degrees C) or hot (46 degrees C) water bath) stimuli were used. Experiments were performed 2 weeks after the surgery when the pain-related behaviour has fully developed. We demonstrated a differential effect of L-365,260 depending both on the dose of morphine and the test used. Pretreatment with the CCK(B) receptor antagonist (0.2 mg/kg) inverted the ineffectiveness of the lowest dose (0.1 mg/kg i.v.) of morphine against the noxious (46 degrees C) thermal stimulus, and the effect of the combination was equal to that seen after the dose 0.3 mg/kg of morphine alone. Likewise, in the mechanical test, the already enhanced effect of this dose (0.1 mg/kg) of morphine on the nerve-injured paw was further increased (by 4-fold) after L-365,260 pretreatment. These effects were abolished by naloxone (0.01 mg/kg i.v.). However, the effects of the higher doses (0.3 and 1.0 mg/kg i.v.) of morphine against the mechanical or noxious thermal stimuli were not significantly enhanced by pretreatment with the CCK(B) receptor antagonist. Further, L-365,260 was found to be completely ineffective in modulating the responses to morphine at 10 degrees C and at 44 degrees C.

Association of enkephalin catabolism inhibitors and CCK-B antagonists: a potential use in the management of pain and opioid addiction

The overlapping distribution of opioid and cholecystokinin (CCK) peptides and their receptors (mu and delta opioid receptors; CCK-A and CCK-B receptors) in the central nervous system have led to a large number of studies aimed at clarifying the functional relationships between these two neuropeptides. Most of the pharmacological studies devoted to the role of CCK and enkephalins have been focused on the control of pain. Recently the existence of regulatory mechanisms between both systems have been proposed, and the physiological antagonism between CCK and endogenous opioid systems has been definitely demonstrated by coadministration of CCK-B selective antagonists with RB 101, a systemically active inhibitor, which fully protects enkephalins from their degradation. Several studies have also been done to investigate the functional relationships between both systems in development of opioid side-effects and in behavioral responses. This article will review the experimental pharmacology of association of enkephalin-degrading enzyme inhibitors and CCK-B antagonists to demonstrate the interest of these molecules in the management of both pain and opioid addiction.

Antiallodynic effects of a CCKB antagonist in rats with nerve ligation injury: role of endogenous enkephalins

Cholecystokinin (CCK) may act as an endogenous anti-opioid and blockade of CCK receptors can enhance the potency and efficacy of morphine. This effect is blocked by opioid delta (delta) receptor antagonists, suggesting a tonic inhibitory action of CCK to diminish the release and/or availability of endogenous enkephalins. The present studies have further evaluated this possibility by studying the antiallodynic actions of a CCKB antagonist (L365,260) alone, or in the presence of thiorphan (a neutral endopeptidase inhibitor) in a model of peripheral neuropathy. Animals subjected to nerve injury, but not sham controls, exhibited long lasting, stable mechanical allodynia. Intrathecal (i.t.) administration of L365,260 or thiorphan alone did not alter allodynia. However, co-administration of these compounds produced a significant antiallodynic action which was antagonized by receptor selective doses of naltrindole, an opioid delta receptor antagonist. In addition, antisera to [Leu5]enkephalin, but not to [Met5]enkephalin, also blocked the antiallodynic action of thiorphan plus L365,260. These data suggest that blockade of CCKB receptors may enhance the actions or availability of endogenous [Leu5]enkephalin or a like substance which can elicit a significant antiallodynic action via opioid delta receptors when its degradation is by inhibited by thiorphan. The data suggest that delta opioids are involved in regulation of some aspects of nerve-injury induced pain.